1. Field of the Invention
The present invention relates to formation of a composite radiographic image from plural radiographic images, and more particularly to conforming the boundaries between the images so as to form a useful composite image.
2. Description of the Related Art
Radiographic imaging, such as x-ray imaging, has long been used for examination and diagnostic purposes. Where successive images are taken in order to provide a continuing view of a subject, it would be beneficial to be able to form a composite image from the successive images.
Prior to the digital age, radiographers manually created a composite image from successive images by laying a portion of one image over another image, attempting to visually align the images, and then taping the images together. In the resulting composite image, there are overlapped portions of the individual images. The composite image was then typically “read” using a light box. However, the overlapped image portions could differ so significantly that the overlapped portion is unusable.
With the advent of digital imaging and digital image processing, a composite digital image can be formed from successive digital images using image processing. The term “stitching” is typically used to refer to the process of melding the digital images to form a single image, and the stitching line refers to the curve at which the images are stitched together.
Techniques are known for stitching together multiple optical images such as digital photographs. A panoramic image can be created by stitching together multiple digital images, for example. Such stitching techniques attempt to blend the images at the seams to provide a “seamless” composite image. A technique for stitching x-ray images is described in International Application No. WO 96/15722, which is discussed below.
A problem that exists when stitching images concerns blending the pixel values (e.g., greylevel pixel values) along a seam or stitching line between adjacent images in order to blend the images. Where the pixel value difference is minimal, blending can be performed with minimal likelihood of introducing erroneous objects, or artifacts, into the image. However, the mismatch between the pixel values of the adjacent images can be such that blending the images becomes difficult and can result in the creation of artifacts. The latter is typically the case with radiographic images, because of the manner in which radiographic images are generated.
In other words, the problems that occur when blending radiographic images are more acute than with optical images, since the contrast (i.e., the degree to which the pixel values differ) between the images can be more extreme due at least in part to the variance in the paths of the rays at the stitching point of adjacent images.
To illustrate, FIG. 1 provides an example of radiographic imaging using x-ray tubes. X-ray tubes 101 and 102 project x-rays onto subject 103 to generate images 104 and 105, respectively. Each x-ray that is projected from one of x-ray tubes 101 and 102 travels in a path (e.g. paths 107 and 108) and encounter subject 103 at a certain angle (e.g., angles α and θ). At stitching line 106, x-ray path 107 encounters subject 103 at angle α, which differs significantly from the angle at which x-ray path 108 encounters subject 103.
The projection of an x-ray via x-ray path 107 through subject 103 is much longer than the projection of an x-ray via x-ray path 108. In addition, x-ray path 107 travels through different portions of subject 103 than x-ray path 108. Consequently, significant contrast difference between the pixel values of images 104 and 105 will likely exist at stitching line 106. As a result, as images 104 and 105 are blended, blending artifacts can be created, which reduce the usefulness of the composite image. For example, if the result of blending images 104 and 105 are used for medical diagnostics, an artifact caused by blending images 104 and 105 may result in an incorrect medical diagnosis.
FIG. 2 depicts a composite image of two adjacent x-ray images both prior to blending and after an unsatisfactory blending of the adjacent images. Composite image 210 includes greyscale images 201 and 202. Along stitching line 200, differences in contrast exist between images 201 and 202. In general, the greylevel pixel values are homogeneously lighter along stitching line 200 in image 201 than the corresponding pixel values in image 202. In addition, region 205 consists of pixels whose greylevel values differ from the values of other pixels along stitching line 200 in image 202, and significantly differ from opposing pixels in image 201 (i.e., region 204).
Composite image 211 was generated using a blending technique, which was determined to be unsatisfactory. According to this technique, all pixels in a column of pixels shared between two images are adjusted based on the difference between pixel values at the boundary between the images.
Composite image 211, which is shown herein to illustrate a result from the technique, consists of images 201 and 202 that are blended by taking an average of the pixels on either side of stitching line 200 and linearly adjusting all of the pixels in a pixel column on both sides of stitching line 200. The pixels are adjusted based on an average of the difference between the pixels at stitching line 200. The number of pixel rows of a shared column that are adjusted is fixed regardless of the pixel values at the stitching line 200.
This blending technique is considered to be unsatisfactory, because artifacts are created as a result, which reduces the usefulness of composite image 211. For example, in order to blend images 201 and 202 at points 204 and 205, the pixels in image 201 are altered to appear darker, and the pixels in image 202 are altered to appear lighter. As a result, the pixels in region 207 are much darker in image 211 than the corresponding pixels in image 210. Similarly, the pixels in region 206 of image 211 are made to appear considerably lighter than the corresponding pixels (i.e., in region 206) in image 210. This results in a banding effect, or artifact, in regions 207 and 206 of image 211.
A fixed blending area is also used in International Application No. WO 96/15722, wherein all of the pixels within an area in which image panes overlap are normalized to blend image panes. Commencing at page twenty, WO 96/15722 describes two different techniques for determining an adjustment amount. In the first, an average is determined by taking the sum of the greylevel values of two overlapping pixels and dividing by two. According to the second described technique, a weighting is applied to the greylevel values of the two overlapping pixels based on the pixel position relative to the two images. In other words, where the pixel is at the side closest to image one, the greylevel value of image one's pixel is given a one-hundred percent weighting, and the greylevel value of image two's pixel is given a one-hundred percent weighting at the side of the overlap region closest to image two.
However, both techniques described in WO 96/15722 use a fixed linear smoothing technique that is applied to a fixed blending area (i.e., the overlap area). Consequently, blending of the image panes is unsatisfactory, since artifacts are likely to result. Further, as indicated on page twenty of WO 96/15722, the technique described therein merely results in less noticeable overlap areas. In other words, the overlap area remains visible after blending, which results in a composite image that is not smoothed across the boundaries of the individual images. In a composite x-ray image, a visible overlap area limits the usefulness of the composite image.
Thus, what is needed is a blending technique that more accurately and responsively takes into account the degree of difference between adjacent images in order to reduce the occurrence of artifacts and to produce more useful composite images.